The present invention is described with respect to its use on riding lawn mowers, particularly self-propelled machines fitted with rotating blades for cutting turf grasses. In the most favored typical design, the rider sits atop a three or four wheeled machine, while one or more blades rotate about a vertical axis within a mower deck mounted at the underside of the machine, to cut grasses as the machine moves across the surface being mowed.
In many typical riding mowers the cutter deck is configured so that it "floats". Typically, a deck is hung between the front and rear wheels and beneath the chassis by chains, links or other devices, so it is adapted to rise up when skids, wheels, rollers and the like attached to the deck underside contact the lawn surface. Generally, the intent for such deck suspension system is to avoid continuing contact with the earth surface; the distance of the cutter deck from the earth surface is determined by the elevation of the chassis. Only when the mower crosses an earth-surface rise which is relatively severe (that is, short in horizontal length compared to the wheel base of the mower and great in height compared to the pre-set elevation of the mower deck) is it expected that the deck will contact the earth surface. Then, it is intended that the deck will rise of "float" upwardly, so the rotary blades will not hit the earth surface. Such designs work well for many kinds of uneveness, but scalping for certain earth surfaces and mower movements is still a problem. And, even if there is no scalping, variation of the height of the cutter deck relative to the earth surface is not wanted, as it varies the height of the cut grass.
Many typical prior art mowers have the wheels rigidly attached to the chassis. Thus, uneveness in the earth surface imparts a lot of up and down chassis motion. Some prior art mowers employ center-pivoting axles which somewhat reduce the vertical motion of the chassis when one wheel encounters uneveness. In an improvement, a lawn mower is fitted with independently suspended wheels, as described in related patent application Ser. No. 09/119,818, discussed further in the Description. The related application describes a preferred transmission steerable mower which has rear drive wheels which are independently driven and spring suspended from the chassis; and, which has free pivoting caster front wheels, mounted at the outer ends of a pivotable axle or subframe. The deck is suspended between the front and rear wheels.
Mowers with improved spring suspension systems reduce the amount of chassis motion when one or both drive wheels of a mower encounter uneveness in the surface being mowed. Drive wheel traction is improved. However, depending on the particulars of any non-rigid suspension system, the chassis is enabled to roll relative to the earth surface; as, for example, when the mower is sharply turning or when the mower is traversing a steep hill side. When a mower rolls, a chassis suspended deck moves closer to the earth surface and there can be a tendency for scalping of the turf by the cutter deck. Thus, there is a need for improvement in the manner in which deck elevation is controlled, particularly for decks used on independent and non-independent spring suspension machines.